Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261805

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
105 PM CDT Wed Oct 26 2016


1110 AM CDT

No major changes to going forecast thoughts, though did tweak
hourly PoP and weather resolution. Lead shortwave coupled with
strong low level jet, despite minimal instability, resulted in
embedded thunder reaching Chicago. General trend should be for
less coverage of thunder this afternoon and gradual sinking
southward of any isolated cloud to ground/in cloud strikes with
instability axis remaining off to our southwest. Respectable
rainfall rates have been occurring with the showers given above
normal PWAT values for late October, but not anticipating any
flooding issues other than minor ponding on some roadways.

Trailing shortwave just ahead of weak surface low pressure and
noted by drying on W/V imagery has resulted in showers filling
back in behind primary rain shield and should see this continue
eastward. Some embedded heavier echoes will result in downpours,
though as mentioned above, expect any lightning to be

Other big story today is the gusty east-southeast to southeast
winds as maximum pressure falls transit the area. This is particularly
along the Illinois shore where coastal platforms have been showing
persistent gusts to around 40 mph. Winds will diminish this
afternoon as gradient weakens with approach of surface low, though
strong winds hanging on longest along immediate IL shore.
Temperatures will be steady to slowly rising through the 40s north
of the low track and exhibit more appreciable rises to the south.
Overall tweaked high temperatures downward some accounting for



258 AM CDT

Through tonight...

Shortwave trough and associated surface low will move across the
region today bringing inclement weather to the area today. Initial
wave of WAA driven showers moving through the area currently
should lift out of the area prior to sunrise. It appears as though
there will be a period of dry weather this morning, lasting
longest over southern portions of the CWA where next wave of
showers may hold off until late afternoon over portions of NW IN
and east central IL. Farther north, more widespread deformation
zone rain should settle ESE into far northern IL this morning and
provide for longer duration steady rainfall. Rain should taper to
a lighter rain or heavier drizzle this evening before ending
altogether by late evening/overnight.

It will be a cloudy day with increasingly strong/gusty winds this
morning, especially northern CWA where pressure falls should be
the strongest. Winds should peak mid-late morning through early
afternoon before tapering off toward evening as baggier gradient
near the surface low approaches. At their peak, winds midday could
gust over 30 mph, with gusts near 40 mph possible along the IL



258 AM CDT

Thursday through Tuesday...

Thursday will be another seasonably cool day with forecast
soundings suggesting we could hang onto a stratocumulus deck a
good portion of the day. Skies should begin clearing Thursday
night as warm air advection begins in advance of the next system.
models continue to track shortwave and fairly strong surface low
quickly east across the U.S./Canadian border Friday then east into
the St Lawrence Valley by Saturday.

Strong warm air advection Friday should allow temps to warm
rapidly into the 60s to near 70 as increasingly strong/gusty
southwest winds develop. Models continue to advertise the
strongest pressure gradient in our area Friday evening with
5-7mb/3hr pressure falls to our north likely to keep winds
strong/gusty despite sunset. Strong low level jet (50-60kt at
925mb) is forecast to be over our area Friday evening. Models
show 2m temps remaining essentially steady or even rising a degree
or two from 00z through 06z Saturday (Friday evening). The result
will likely be continued efficient mixing, which could tap into
some of that higher momentum air of the low level jet. Didn`t get
too carried away in the grids yet, but potential for a period of
near advisory level gusts (45 mph) are possible Friday evening.

The strong winds will keep temps largely steady in the 60s much of
the night Friday, though as front approaches late at night winds
should ease some and probably allow temps to drop off into the 50s
over NW portions of our CWA. Looks like a very mild start to the
day Saturday, but models have trended about 8-10 hours faster with
cold front sagging south across the CWA, which means much cooler
temps Saturday afternoon northern CWA based on current model
progged timing. High temps Saturday will be tricky and hinge on
the exact timing of fropa, particularly near Lake Michigan where
some lake enhancement of the front will know temps down even
farther near the lake.

Latest ECMWF has a shortwave trough riding the WNW flow aloft into
the region Saturday night and breaks out a healthy band of
rain/isolated t-storms north of the now stalled out front across
our CWA. GFS has a similar idea though a bit farther south.
Despite low confidence at this point, have opted to added pops to
the initially dry model blended cocktail grids Saturday night.

After a seasonable temps Sunday, another strong surface low is
forecast to track well to our north Monday. The result will be
another day of strong warm air advection and unseasonably warm
temps on Monday. Medium range models are currently forecasting
850 and 925 mb temps to be near or in some cases a few degrees
above records for this time of year. Have nudged temps up in the
model blend that we initial our forecast with, but remained
conservative with increases as there is still time for models to
slow or speed up the timing the front and warmest air. It is worth
noting that if the warmer solutions pan out, then temps near or
even reaching 80F would be well within reach!! Another weak cold
front is progged to bring an end to the unseasonable warmth Monday
night, though temps look to remain solidly above average into at
least the middle of next week.



For the 18Z TAFs...

* IFR CIGs and IFR to low MVFR VSBY this afternoon through
  evening, with low CIGs lingering into Thursday.
* East-southeast winds shifting north-northwest tonight.
* Periods of light rain/showers and drizzle through this evening.

Weak low pressure currently across southeast Iowa will track
across northern Illinois this evening. Rain showers and drizzle
north of the warm front extending east from the low will continue
this afternoon and transition to the light rain/drizzle being
observed across NW IL for the rest of the area. Rain cooled air is
aiding in quickly lowering CIGs and now anticipate IFR to occur at
all terminals this afternoon. Cannot rule out LIFR, particularly
at RFD. The low clouds should stick around on Thursday, with
bases rising to MVFR. Confidence in overall CIG trends and timing
is medium. Will amend as needed to refine timing for late
afternoon/evening rush.

Of additional concern is the low VSBY with the showers and
especially in the drizzly regime in northwest Illinois that should
also translate east-southeast. Some of the guidance is hitting
low VSBY fairly hard tonight. TAFs include IFR VSBY at RFD, with
low MVFR at other sites, but may also need to make adjustments
with only low-medium confidence. Southeast winds 10-15 kt with
higher gusts will trend to more east-southeast 10 kt or so north
of the low path this evening. Then after some possible variability
in immediate vicinity of low center, winds will shift to NW/NNW,
with NW winds continuing on Thursday. This wind shift should also
scour out the lower VSBY overnight. Have medium-high confidence in
easterly wind component lingering at ORD/MDW through the evening



353 AM CDT

Main concerns are with the expected gales across much of the lake
this morning through the afternoon and early evening. Low pressure
over Nebraska and Iowa will continue to push northeast towards the
southern end of the lake today into tonight and as this occurs,
expect southeast winds to quickly increase this morning. Gales to
35-40 KT are likely this morning through the afternoon for the
nearshore waters and south half of the lake, while gales continue
to be a possibility over the north half. Have not issued any
headlines over the north half, but will need to continue to
monitor for the possibility of gales in this location later this
morning. Gales will then likely be focused only over the open
waters later this evening, before diminishing with the eastward
shift of this low pressure system. With gales diminishing, higher
waves hazardous to small craft will likely linger for a period
tonight into Thursday. Next focus of concern will be Friday
afternoon into Friday night as another area of low pressure moves
through the region. Windy conditions are appearing likely, with
gales becoming more probable over much of the lake during that



LM...Gale Warning...LMZ777-LMZ779 UNTIL 10 PM Wednesday.

     Gale Warning...NEARSHORE WATERS UNTIL 7
     PM Wednesday.




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